In the drilling and completion of oil and gas wells, a cementing composition is often introduced into the wellbore for cementing pipe string or casing. In this process known as ‘primary cementing’, a cement slurry is pumped into the annular space between the walls of the wellbore and the casing. The cement slurry sets in the annular space, supporting and positioning the casing, and forming a substantially impermeable barrier or cement sheath, which isolates the wellbore into subterranean zones to prevent undesirable migration of fluids between such zones.
There are two main factors that contribute to ensuring zonal isolation during the life of a well. Specifically the cement should be placed in the entire annulus through efficient mud removal and the properties of the set cement should be optimized so that it can withstand the stresses from various operations that may be conducted during the life of the well.
If the short term properties of the cementing composition such as density, static gel strength, and rheology are designed as needed, the undesirable migration of fluids between zones is prevented immediately after primary cementing.
Poor zonal isolation results in fluid migration, e.g. water or gas may invade an oil-bearing zone, resulting eventually in a risk of blow out, or to a less severe but economically challenging problem such as water production (and the need to provide expensive water treatment surface facilities) or the loss of reserves and productions. Remedial work to repair a faulty cement job is expensive (in as much as it increases rig time and delays oil or gas production) and sometimes leads to irreparable harm to the hydrocarbon-bearing production.
In most cases poor zonal isolation results from poor mud removal. As the cement slurry rheology is a critical parameter in the process of mud displacement, the slurry should have a constant rheology when it is prepared and as it is being pumped into the well and placed in the area that is to be cemented. However as the temperature of the well increases with depth, the rheology of the cement slurry will change with the temperature changes.
Most of the polymers that are currently used in cement slurries are either water-soluble polymers that dissolve quickly at ambient temperatures or non-soluble polymers like latexes. In both cases when the bottom hole circulating temperature is significantly higher than ambient temperature the rheology of the cement slurry decreases significantly during the placement of the cement. The rheology of the slurry can be increased by adding more polymer so that the cement slurry will still have sufficient viscosity when exposed to the higher temperatures downhole. However this can lead to mixing problems when preparing the cement slurry composition due to the slurry then having very high viscosity at ambient temperatures.
Therefore it is an object of the invention to provide an additive for a cementing composition that will maintain a constant rheology independent of the temperature that the slurry is exposed to.